


From Alpha To Omega

by reaperzzz



Category: Undertale (Video Game)
Genre: Alpha/Beta/Omega Dynamics, Alpha/Omega, Dirty Talk, Dirty Thoughts, F/M, Impregnation, M/M, Marking, Mating Cycles/In Heat, Mildly Dubious Consent, Multiple Orgasms, Overstimulation, POV Multiple, Rough Sex
Language: English
Status: In-Progress
Published: 2018-04-07
Updated: 2018-04-07
Packaged: 2019-04-19 15:43:32
Rating: Not Rated
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 3,521
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/14240526
Author URL: https://archiveofourown.org/users/reaperzzz/pseuds/reaperzzz
Summary: Swapfell Sans is mortified when he goes into heat. Normally, he wouldn't care about this sort of thing, but his normally Alpha heat was replaced with an Omega heat. He is outright determined to not let his mutt know about this. But life has a funny way of working out, doesn't it?





	From Alpha To Omega

**Author's Note:**

  * For [MeGaLoTrash](https://archiveofourown.org/users/MeGaLoTrash/gifts).
  * Inspired by [Honeypot](https://archiveofourown.org/works/13974102) by [MeGaLoTrash](https://archiveofourown.org/users/MeGaLoTrash/pseuds/MeGaLoTrash). 



Black whimpered to himself, softly saying every swear word he could think of. Why. Why. WHY. Why did someone as Mighty and Terrifying as The Malevolent Sans have to be reduced to a squirming begging mess??? He was an Alpha damn it!

 

So why for the love of Queen Toriel was he experiencing an Omega heat?! _Why did he want Papyrus to bend him over to kitchen table and fuck his brains out, filling his tight pussy up and impregnating him-_ He growled at his returning thoughts, his face a dark blue.

 

He also hated the fact he couldn't ask the bitch of the Royal Scientist about this predicament, because the only other person who would know anything about these Alpha, Beta, and Omega roles skeletons go through was his smug asshole brother, Slim. And no way in the Underground was he telling him about this.

 

He uncurled from his ball and tried to walk to his training grounds like he usually did. But with every step his body felt like it was burning hot with need, getting to a maddening point. He groaned and knew he'd have to come up with an excuse to talk to Slim about this. "I Can Deal With This Until I Get Home....."

 

He quickly walked to where Alphys was impatiently tapping her foot "There you are! Hurry up and get ready, we're behind on schedule!" He rolled his eyes like he normally did at her brash and loud nature. He nearly vomited when the thought of such a dominant woman putting him down where he belonged popped into his head.

 

He shuddered in disgust at that thought. ' _No, no, she can't be my mate! She's not even my own kind!_ ' Why was he even entertaining this disgusting thought?? He summoned his bones and attacked her with them as he normally did. But everything seemed to go wrong today.

 

His aim was off, he barely hit her, and he couldn't dodge for shit. What would usually be just the beginning of the training turned out to be the end as Alphys quickly walked up to him "What's gotten into you, Black??" He shooed her away with his hand weakly "Oh Shut Up, I'm Perfectly Fine."

 

Alphys scoffed "You can barely stand." He glared at her "I'll Break Your Damn Legs If You Don't Shut Up" Alphys raised her eyebrow in amusement "Oh? Because the only thing you look like you're gonna do is collapse. I thought I trained you better than this, Black.."

 

Black knew she was baiting him. While one side of him was saying to retaliate and prove he wasn't useless, the other painfully annoying side was coaxing him to apologize to her and beg for forgiveness for being so weak and helpless. He summoned sharp bones and threw them all at her, giving her plenty of scratches and bruises

 

"There's the Black I'm used to!" She said with a wicked smile, swinging her axe at him wildly. He dodged and shot her axe clean in half with bone going at lightening speed. He was very obviously trying to over compensate for the fact his growing heat was turning him softer.

 

After everything was done, they were both panting and covered in injuries. Nothing too serious, but Alphys was a bit surprised. "What's with you, man? First you come here late, which never happens, then you fall behind on training, now your better than ever! I can never figure you out.."

 

Black simply shrugged and dusted himself off. Alphys stretched and popped her joints, waving goodbye to him "See ya later, buddy" Black didn't respond and jogged home. He had to take care of this heat, NOW. The only reason he'd upped his game back there was so that he could get home faster.

 

He locked himself in his room, closing all his curtains. He'd get around to checking on Slim later, right now, he'd focus on the throbbing in between his legs. He grimaced as he knew what his body had treacherously formed, but it still gave him a bit of shock when he saw it. 

 

As he stripped off his clothes slowly, his dark blue soaking wet pussy came into view. And the smell that came from it... He screamed into a pillow in embarrassment. The smell was so inviting and sweet and delicate! Nothing like him at all! He sighed angrily and grabbed a box from under his bed.

 

He normally kept these toys for Slim, but unfortunately, that mutt of a brother of his wasn't the one who needed or wanted them. He shyly lined the toy up with his weeping entrance. He gasped and felt his face light up more when his disgusting body formed a full female ecto-body!

 

He groaned loudly in annoyance. This would be way harder to hide now! "Stupid body..!" He slipped the tip of the toy inside his vagina, letting out breathy moans and biting down on his scarf. This felt so good! He'd needed this type of gratification all day long!

 

He gripped his scarf and pushed the rest of the toy inside of him, gasping as his legs spasmed randomly. His pussy was much too sensitive to handle this, and it squirted out his cum, staining his once pristine white sheets. He eagerly started thrusting the toy in and out, getting lost to his heat.

 

"Ah! Yes! Oh fuck yes, baby!~" He was both sickened and loving of this whole new role. It was so rewarding! _Would you seriously let someone as Great and Mighty as you get dominated by a measly low life???_ His prideful and logical side tried to convince him to ignore this and stop.

 

But he wasn't just wanting this now, he craved required this like his damn life depended on it. He moaned and squealed into his scarf, jolting and twitching as he hit all the good spots inside his pussy. He laid back on his bed, his legs up in the air as he just pounded his pussy. 

 

He came more and more frequently, but the more he came, the more he knew he needed more than this. And the only person to do that was- He suddenly snapped out of his pleasure enduced craze "Fuck No! I Am Not Letting This Stupid Heat Take Any More Control Over Me!"

 

He cleaned off the toys and put them back into his box, stuffing it far under the bed. He got on his clothes shakily, hating that his weak legs started to tremble when he put too much pressure on them (aka when he crouches or bends his knees) He walked quickly down the stairs, and out the door to check up on his good for nothing brother.

 

He felt his breasts gently sway as he speed walked over to Slim's sentry station. "Deform, Damn It...!" He whisper yelled to himself. His breasts didn't seem too big, thankfully, so his shirt and chest plate mostly hid them. He found Slim as he normally did, fast asleep with drool covering his desk slightly.

 

He scowled in detestment and slammed his hand down on the desk, waking up Slim quickly "What do I have to do to keep your sloppy ass awake for one god damn shift?!" He swore profoundly in his head as the mere presence of his lazy messy brother made his heat go insane.

 

He smirked and shrugged, stretching and lighting a cigarette "i dunno, bro. i'm figurin that out too..." Black rolled his eyes "It's Really Sad Seeing You Constantly Fail At Your Job, You Know.. It Makes Me Worry About You..." Fuck. 

 

That sounded way softer than he intended it to be like. He sounded so timid and reserved. He quickly recovered from the seemingly mutual shock from the two brothers "Sad As In Disappointing! I Can't Believe I Have To Do Twice The Work To Support Us Just Because Your Lazy Ass Can't Work For Shit!"

 

And now he just sounded like an angry wife, yelling at her husband. Ugh. Better than the shy tone he'd acquired earlier on accident. Slim snickered at his stumbling brother. "you doin ok, bro? ya sound flustered~" He said in a flirty tone. No, no, no, he did NOT need that right now.

 

His heat had been making the smell from earlier come back twice as hard, his body now radiating with the smell of sweet flowers and raspberries. He definitely didn't need Slim's mindless flirting making this worse. He growled at him "I Am Not Flustered, You Mutt!! I'm Going Home To Cook Dinner, Don't Fall Back Asleep!"

 

"what are ya gonna do if I do fall asleep, hmm?" Black turned around and stomped over to him, grabbing him by his shirt collar. "If You Slack Off Again, I Swear To Queen Toriel I'm Going To Wipe That Smug Grin Off Your Face.." Slim snorted and wiggled out of his grip.

 

"sure you will, bro." Black decided to not intervene anymore and go home to cook his signature enchiladas. He felt his soul beat like crazy as he remembered the encounter with his normally compliant brother. Who the hell did Slim think he was, talking to Black like he was a child trying to intimidate him?!

 

 _Maybe I should just let go, and let Slim dominate me... To feel his strong arms pin me down and for him to mark me as his own..._ Black face palmed himself as he shoved those fantasies away. He. Was not. Ever. Going to let that happen to him..! He heard the door open up, and snapped his head towards the door.

 

"What Are You Doing Home, Slacker?! You're Not Off Your Shift Yet!" Slim had a sleazy grin on his face. That can't be good... Slim gently cupped his brothers face, kissing his forehead, which made Black barely conceal a soft moan with a growl. "ya know, the weirdest thing happened today while you were around..."

 

Black tried to push Slim off of him, his body pressed up way too close against Slim's. If he was too close, he could find out about all the things he'd rather not have to explain and probably show to his brother. "W-What Are You Talking About, Slim?" Black desperately tried to sound natural as Slim purposely or accidentally grinded against him.

 

Slim smirked down at him "well, when you came by, a sweet smell suddenly appeared out of nowhere. i thought it was something nearby, since you never smell sweet. but the only thing close enough to me for it to smell as strongly as it did was you and my desk, and the smell only showed up when you were around.."

 

Black blushed darkly as Slim started to piece together the pieces. He nervously glanced away "That Wasn't Me, Idiot. It Must've Been That Honey You Constantly Drink" Black said, quickly finding an excuse. Which was soon crushed by Slim, yet again

 

"nah, that can't be it. my honey doesn't smell like rasberries, and not so strongly too... it was like someone shoved raspberry perfume in my nose." Slim leaned down, and sniffed Black's neck. Shivers went down Black's spine, making him more and more prone to blushing.

 

"What Are You Doing?! Don't Sniff Me!!" Black pushed him away, pouting without realizing it. Slim purred "no doubt now, you smell like that. only question left is... why? unless something is...off about you today, you shouldn't smell so dainty and feminine" Black looked at him angrily, hating he figured this out so fast.

 

"Look, I Don't Know Why I Smell Sweet Or Feminine, But Just Because I Smell Good Doesn't Mean I'll Start Acting Nicer" Slim raised his eyebrow "oh really?" He pinned his brother to the table, his hands holding Black's down firmly. Black attempted to escape but to no avail.

 

"but wouldn't you like that? don't you want to be the sweet, kind, gentle person I love?" Black's soul flaired up and his carefully chosen words. No doubt, Slim knew what had happened to him, and was trying to get him to give into the heat. Well now way that was ever happening!

 

"Like Hell I'll Do That!" Black winced at his tone _Shh, quiet down, your mate is right here... Just be quiet and submissive for him, he'll like that.._ The annoying thoughts came back, making his desires stronger than before again. Slim gently placed kisses all over Black's neck, running his hand down his sides.

 

Black suppressed a whine of embarrassment as Slim sucked on his neck "don't lie to yourself, dear... i know you want to just be nice and submissive for your mate, don't you?~ this heat is only bringing out the side of you you've so desperately tried to hide over the years..."

 

Black panted softly and shakily at his brothers words and movements. Slim pulled away, looking Black in the eye. Black knew that look in his eye. It was the same look he gave Slim every time they fucked. It was a look that meant the poor sub was gonna have a long night.

 

Black pouted and weakly tried to push Slim away again, being easily stopped by Slim pushing his body against the...bed? Black looked around, noticeably frightened. He forgot for a moment his brother could teleport. "P-Please...Brother....Don't..." Black said as a last attempt. This would damage his ego and reputation so much.. 

 

He couldn't live with himself if he allowed his brother to take him.. It was his job to be dominant, not Slim's! He whimpered and looked away, tears forming in his eyes. Slim kissed him gently, taking off Black's pants. Black squeaked and covered up, squeezing his legs closed as tight as he could.

 

Slim rubbed his inner thigh, coaxing him to open up, both physically and emotionally. "Slim... This Isn't Supposed To Happen.... This Is Wrong... I Feel So Helpless and Submissive..." Slim held his hand, kissing his jawline "that's how omegas are, dear.. they're submissive and crave for a fitting mate to have children with~"

 

Black whined again "B-But... I Can't... I Won't Be... I Can't Be Weak Like That... I'll Be A Laughing Stock..." Black seemed to be so reliant on other's opinions on him. Of course that made sense in the world, since any show of weakness is like holding up a giant sign that said "FREE EXP!" And pointing it towards yourself.

 

But Slim had to break him out of that mindset. For this to work, Black actually had to trust Slim enough to give him the control and power over his sensitive heat polluted body. "brother, please, listen to me." Slim took his head and turned it towards him.

 

"you have to let go of your fear of being weak.. for this to work, you have to listen to your heart, and not your mind. i promise, this will feel so good.. you'll feel so loved and wanted, your legs will look amazing around my waist~" Black's breath hitched at the last sentence.

 

Black felt his resolve slipping away, starting to let his heat take over. He shyly kissed Slim, slowly wrapping a leg loosely around Slim's lower back. His pussy was easily visible in this position, prompting Slim to pull down his pants and let his throbbing hard cock free.

 

Black softly gasped, biting his lip. "You're So Big.." Black looked away before taking off his chest plate and shirt, his breasts being revealed. He covered his face with his scarf, embarrassed beyond words at this point. The room was soon filled with the sweet inviting smell Slim was growing used to.

 

Said skeleton cooed and kissed down Black's chest, sucking on his nipple. Black cried out in pleasure and shock "Be Careful, M-Moron!~ They're Very Sensitive!~" Slim had an annoyed look on his face "is that anyway to talk to me?" 

 

Black quickly apologized, kissing his forehead "I'm Sorry, I'm Just Used To Doing That..." Slim kissed Black's puffed up areolas and smiled at him "you're so cute, dear... this feels so right to me, with you moaning under me~" Black shyly nodded in agreement "This Does Feel Oddly Natural.."

 

Slim kissed down his skinny stomach to his inner thighs, which were covered in the sweet juices coming out from Black's begging pussy. Slim licked up the juice from Black's leg, causing him to squeal and cum, saying a slew of apologies for cumming so early.

 

Slim kissed his clit and put his legs on his shoulders. "you taste so sweet and pure, dear~" Slim sucked the juices off of Black's drenched clit, but as soon as the juices were cleaned off, Black's pussy soon replaced it with more. Slim licked up his whole vagina, paying attention to the spots Black seemed to like having extra attention on.

 

Black's legs spasmed and trembled as his fragile pussy was being roughly eaten out by his mate. He grabbed onto Slim's skull, squirting into his mouth again. He arched his back and cried out Slim's name. Slim kissed his vagina through his orgasm.

 

Slim pulled away from his heeping form for a couple seconds, lining up his length with Black's pussy. He nuzzled his tip in between Black's lips, rubbing the head of his hard cock against Black's stretched entrance. Slim wasn't angry and knew Black used sex toys at first to try and get rid of his omega heat.

 

"you ready?" Black nodded and thrusted his hips up into Slim's, making half of Slim's length go inside his tight sopping wet vagina. "shit, you're cunt is so tight around my cock~ how much have you fantasized about this today?~" Black blushed more but answered quietly "More times than I'd like to admit..."

 

Slim held his hips up high, thrusting in his entire length harshly. Black had a small orgasm from the feeling of being so full. He wrapped his legs tightly around Slim's waist, bucking his hips "P-Pound Me Until My Legs Are Like Jelly~" Black softly whispered into Slim's ear.

 

Slim nodded and started to roughly thrust into Black. He let out loud and high pitched moans as his cute pussy was being harshly penetrated by his newly found mate. Slim went harder and faster it seemed with every thrust, making Black have the time of his life.

 

His eyes were rolled back to the top of his skull as he clawed desperately into Slim's back, begging for nothing. His legs were sore from Slim's brutal treatment, but he couldn't give less fucks about that. His mind was only focused on two things. 

 

Slim's cock going in and out of his soon-to-be-breed pussy, and the dominating and musky scent of Slim that was driving him crazy with lust. Black wanted to constantly have that smell surrounding him, everywhere he went. It made him feel safe and secure, while also feeling so hot and horny...

 

Both of the two skeletons soon came close to their peaks, their moans and sounds getting louder. Black drooled slightly as Slim just fucking annihilated his poor vagina. Slim swore softly under his breath "shit, sweetheart, i'm close~ but i'm guessing that's a good thing, considering how much you're clamping around my cock"

 

Black could only let out loud moans and near screams of pure ecstasy. He arched his back high as dark blue cum shot out of his sore and stretched hole. This prompted Slim to fill Black up with his creamy semen, gripping his hips tightly. Black panted heavily, tears of overstimulation falling down his face.

 

Slim cuddled up with Black, pulling the blanket over them. Black snuggled into Slim's chest, purring as he felt cum slowly drip down his thighs "I Love You..." Slim kissed his forehead gently "i love you too, dear.." Black yawned and fell asleep, Slim soon following along.

 

 

 

 

 

3313 words... Wow. Didn't expect this to be that long, but, then again, I'm actually glad I wrote this much. If anyone is confused on the whole A/O stuff in this, here's how I imagine it works (in this fic)

 

When skeletons have matured enough, getting past their wild early life stages and moving more into their 'settle down' phase, their bodies start to change their Alpha, Beta, and Omega role one final time. The weaker ones get either Beta or Omega, while the strong ones get assigned the Alpha role.

 

Although Black is very strong, Slim is much stronger, but he just hides it from everyone, and passes himself off as a weakling to avoid any hard work. The second/last change is barely focused on how the person feels, and rather on their physical strength. 

 

Since Black and Slim were the only two skeletons left in their universe, Black had to go through the process of being an Alpha to an Omega. If there was another skeleton in their universe, the probability of them becoming the Omega instead would be around 70-80%, and Black instead would've just become a Beta.

 

So that's my explanation for this fic's logic, sue me, I'm done

 

I might continue this or just leave this as a one shot


End file.
